SOI CMOS device with reduced DIBL

ABSTRACT

A CMOS device formed with a Silicon On Insulator (SOI) technology with reduced Drain Induced Barrier Lowering (DIBL) characteristics and a method for producing the same. The method involves a high energy, high dose implant of boron and phosphorus through the p- and n-wells, into the insulator layer, thereby creating a borophosphosilicate glass (BPSG) structure within the insulation layer underlying the p- and n-wells of the SOI wafer. Backend high temperature processing steps induce diffusion of the boron and phosphorus contained in the BPSG into the p- and n-wells, thereby forming a retrograde dopant profile in the wells. The retrograde dopant profile reduces DIBL and also provides recombination centers adjacent the insulator layer and the active layer to thereby reduce floating body effects for the CMOS device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor devices andfabrication processes and, in particular, to CMOS devices formed in asilicon-on-insulator (SOI) technology with reduced drain induced barrierlowering (DIBL) and a method for fabricating the same.

2. Description of the Related Art

There is an ever-present desire in the semiconductor fabricationindustry to achieve individual devices with smaller physical dimensions.Reducing the dimensions of devices is referred to as scaling. Scaling isdesirable in order to increase the number of individual devices that canbe placed on a given area of semiconductor material and to increase theprocess yield and to reduce the unit cost of the devices and the powerconsumption of the devices. In addition, scaling can result inperformance increases of the individual devices as the charge carrierswith a finite velocity have a shorter distance to travel and less bulkmaterial has to accumulate or dissipate charges. Thus, the trend in theindustry is towards thinner device regions and gate oxides, shorterchannels, and lower power consumption.

However, scaling often creates some performance drawbacks. Inparticular, a known category of performance limitations known as shortchannel effects arise as the length of the channel of CMOS devices isreduced by scaling. One particular short-channel effect in CMOS devices,known as Drain Induced Barrier Lowering (DIBL) is mainly responsible forthe degradation of sub-threshold swing in deep submicron devices. DIBLis a reduction in the potential barrier between the drain and source asthe channel length shortens as illustrated in FIG. 1 reflecting knownprior art. When the drain voltage is increased, the depletion regionaround the drain increases and the drain region electric field reducesthe channel potential barrier which results in an increased off-statecurrent between the source and drain.

In conventional CMOS devices, a retrograde channel dopant profile can beeffectively used to control DIBL. In a CMOS process, n-type and p-typewells are created for NMOS and PMOS devices. In a typical diffusionprocess, dopant concentration profiles in these n- and p-type wells areat a peak near the surfaces and decrease in the depth direction into thebulk as illustrated in FIG. 2. A retrograde profile is one in which thepeak of the dopant concentration profile is not at the surface but atsome distance into the bulk as shown in FIG. 3. Such retrograde profilesare helpful in deep submicron CMOS devices since they reduce thelowering of the source/drain barrier when the drain is biased high andwhen the channel is in weak inversion. This limits the amount ofsubthreshold leakage current flowing into the drain. A lower level ofsubthreshold leakage current provides improved circuit reliability andreduced power consumption.

A retrograde dopant profile also typically results in a lower dopantconcentration near the surface of the wafer which reduces junctioncapacitances. Reduced junction capacitances allow the device to switchfaster and thus increase circuit speed. Typically, retrograde profiledopant implants are done after formation of the gate. A halo (or pocket)implant is another known method used in deep submicron CMOS devices toreduce DIBL.

However in some applications, such as in an SOI process, it is difficultto create a retrograde profile due to the thinness of the silicon layerand the tendency of the dopants to diffuse. A SOI process has a buriedinsulating layer, typically of silicon dioxide. State-of-the-art SOIdevices have a very thin silicon (Si) film (typically <1600 Å) overlyingthe oxide in which the active devices are formed. Increasing the Si filmthickness any further will increase the extent to which the devicesformed therein get partially depleted. SOI devices also suffer from‘floating body’ effects since, unlike conventional CMOS, in SOI there isno known easy way to form a contact to the bulk in order to remove thebulk charges.

When the as-implanted retrograde dopant profiles diffuse duringsubsequent heat cycles in a process, they spread out and lose their‘retrograde’ nature to some extent. In SOI, since the silicon film isvery thin, creating a true retrograde dopant profile is very difficult.This is true even while using higher atomic mass elements like Indium(In) for NMOS and Antimony (Sb) as channel dopants. Diffusivity of thesedopants in silicon is known to be comparable to lower atomic masselements like boron (B) and phosphorus (P), when the silicon film isvery thin, as in an SOI technology. Moreover, leakage current levels areknown to increase when Indium is used for channel dopants (See “Impactof Channel Doping and Ar Implant on Device Characteristics of PartiallyDepleted SOI MOSFETs”, Xu et al., pp. 115 and 116 of the Proceedings1998 IEEE International SOI Conference, October 1998 and “DopantRedistribution in SOI during RTA: A Study on Doping in Scaled-down SiLayers”, Park et al. IEDM 1999 pp. 337-340, included herein byreference).

From the foregoing it can be appreciated that there is an ongoing needfor a method of fabricating deep submicron SOI CMOS devices whileminimizing short channel effects such as DIBL. There is a further needfor minimizing DIBL in deep submicron CMOS devices without incurringsignificant additional processing steps and high temperature processing.

SUMMARY OF THE INVENTION

The aforementioned needs are satisfied by the SOI CMOS device withreduced DIBL of the present invention. In one aspect, the inventioncomprises a semiconductor transistor device comprising: a semiconductivesubstrate; an insulative layer buried within the semiconductivesubstrate; an active layer of semiconductive material above theinsulative layer; a plurality of doped device regions in the activelayer; a gate structure formed on the device regions; source and drainregions formed in the device regions such that the doping type for thesource and drain is complementary to the doping type of thecorresponding device region; dopant diffusion sources placed within theburied insulator layer underlying the device regions wherein the dopantdiffusion sources diffuse into the device regions so as to create aretrograde dopant profile in the device regions; a plurality ofconductive layers electrically interconnecting the transistor devices;and a passivation layer overlying the conductive layers. In oneembodiment, the semiconductive substrate, insulative layer buried withinthe semiconductive substrate, and the active layer of semiconductivematerial above the insulative layer comprise a SOI Separation byIMplanted OXygen (SIMOX) wafer.

Another aspect of the invention comprises dopant atoms implanted throughthe device regions such that the dopant atoms come to reside within theBuried OXide (BOX) layer underlying the device regions creating aborophosphosilicate glass (BPSG) within the BOX layer. Formation of thepassivation layer causes the dopant atoms contained within the BPSG todiffuse into the device regions so as to create the retrograde dopantprofile in the device regions. The retrograde dopant profile has a peakconcentration substantially adjacent the interface of the BOX and theactive region. The retrograde dopant profile in the device regionprovides the transistor device with improved resistance to drain-inducedbarrier lowering (DIBL) and also provides the transistor device withrecombination centers to reduce floating body effects.

In another aspect, the invention comprises a method for creatingsemiconductor transistor devices comprising the steps of: providing asemiconductor substrate; forming a buried insulation layer in thesemiconductor substrate; forming an active layer above the buriedinsulation layer by placing additional semiconductor material on theburied insulation layer; doping the active layer with dopant atoms so asto form device regions; implanting additional dopant atoms through thedevice regions such that the additional dopant atoms come to residewithin the buried insulation layer underlying the device regions;implanting dopant atoms into gate regions of the device regions; forminga gate stack on the active layer adjacent the gate regions; implantingdopant atoms into the device regions such that the dopant atoms come toreside within the device regions adjacent the gate regions so as to formsource and drain regions and wherein the gate stack substantiallyinhibits penetration of the dopant atoms into the gate regions of thedevice regions; forming conductive paths that electrically connect tothe source, drain, and gate regions; and forming a passivating layeroverlying the conductive paths. The method of the invention alsoincludes implanting dopant atoms through the device regions wherein thedopant atoms come to reside within the BOX layer underlying the deviceregions thereby creating a borophosphosilicate glass (BPSG) within theBOX layer.

In another aspect of the invention, formation of the passivation layerinduces the dopant atoms contained within the BPSG to outdiffuse intothe device regions thereby forming a retrograde dopant profile withinthe device regions. The retrograde dopant profile within the deviceregions reduces DIBL effects for the CMOS device and also providesrecombination centers adjacent the BOX active region interface therebyreducing floating body effects. These and other objects and advantagesof the present invention will become more fully apparent from thefollowing description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating prior art concerning DIBL as the relationof threshold voltage (V_(T)) to drain-source voltage (V_(DS)) forvarious sub-micron channel lengths;

FIG. 2 is a graph illustrating prior art of a typical diffusion baseddopant profile in CNOS devices;

FIG. 3 is a graph illustrating prior art of a retrograde dopant profilein CMOS devices;

FIG. 4 is a section view of the starting material of the SOI CMOS withreduced DIBL, a SIMOX wafer;

FIG. 5 is a section view of the SIMOX wafer with n- and p-type wellsformed therein and a high dose, high energy implant into the buriedoxide (BOX) forming a borophosphosilicate glass (BPSG) structure;

FIG. 6 is a section view of the SIMOX wafer with gate stacks formed onthe n- and p-wells with source and drain implants;

FIG. 7 is a section view of the SOI CMOS devices with conductive andpassivation layers in place with the dopants entrained within the BPSGoutdiffused into the n- and p-wells thereby forming a retrograde dopantprofile within the wells that reduces DIBL;

FIG. 8 is a graph illustrating the net dopant concentration in thechannel (gate) region of a SOI CMOS of the present invention as afunction of depth into the substrate from the surface to the buriedoxide layer; and

FIG. 9 is a graph illustrating the dopant concentration in thesource/drain regions of a SOI CMOS of the present invention as afunction of depth in the substrate from the surface to the buried oxidelayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made to the drawings wherein like numerals referto like structures throughout. FIG. 4 is a section view of oneembodiment of the SOI CMOS with reduced DIBL 100 of the presentinvention showing the starting SOI material, a Separation by IMplantedOXygen (SIMOX) wafer 102. The SIMOX wafer 102 is well known in the artand comprises a silicon substrate 104 in which a layer of the substrate104 is converted to a buried silicon dioxide (BOX) 106 layer with aheavy oxygen implant and subsequent anneal. An epitaxial layer 110 of Siapproximately 500 Å to 2500 Å thick is then grown on top of the BOXlayer 106. The BOX layer 106 of the SIMOX wafer 102 provides electricalinsulation between the active region of the epitaxial layer 110 and thebulk silicon of the substrate 104. Thus, active devices formed in theepitaxial layer 110 are electrically isolated from the semiconductivesubstrate 104. The SIMOX wafer 102 also provides physical structure aswell as reactive material for formation of the SOI CMOS with reducedDIBL 100 in a manner that will be described in greater detail below.

In the description of the SOI CMOS with reduced DIBL 100 that follows, asingle CMOS 130 structure comprising PMOS 132 and NMOS 134 (FIG. 7)devices will be used to illustrate the invention. It should beappreciated that the process herein described for one CMOS 130 devicealso applies to forming a plurality of SOI CMOS with reduced DIBL 100devices. It should also be appreciated that the invention hereindescribed can be modified by one skilled in the art to achieve a PMOS132, an NMOS 134, or other technology employing the methods hereindescribed without detracting from the spirit of the invention. It shouldalso be understood that FIGS. 4-7 are illustrative and should not beinterpreted as being to scale.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisescreating n-well 112 and p-well 114 regions as shown in FIG. 5. Then-well 112 and p-well 114 regions are created, in this embodiment, byimplanting a dose of approximately 1e13/cm² of P @ 60 keV to create then-well 112 and a dose of approximately 1e13/cm² of B @ 30 keV to createthe p-well 114. The n-well 112 and p-well 114 are then driven at atemperature of approximately 800° C. for a period of approximately 30minutes. The n-well 112 and p-well 114 provide regions for thesubsequent formation of the PMOS 132 and NMOS 134 devices that comprisea CMOS 130 device (FIG. 7).

The method of forming the SOI CMOS with reduced DIBL 100 then compriseshigh energy, high dose n-type diffusion source 116 and p-type diffusionsource 120 implants into the p-well 114 and n-well 112 respectively asshown in FIG. 5. The n-type diffusion source 116 and p-type diffusionsources 120 comprise borophosphosilicate glass (BPSG). The n-typediffusion source 116 and p-type diffusion source 120 implant parametersshould be tailored in such a way that the resultant n-type diffusionsource 116 and p-type diffusion source 120 dopant profiles mainly residein the BOX layer 106. In one embodiment, the n-type diffusion source 116implant comprises an implant of phosphorus through the n-well 112 ofapproximately 2.0e14/cm² @ 220 keV into the BOX layer 106 and the p-typediffusion source 120 implant comprises an implant of boron through thep-well 114 of approximately 2.0e14/cm² @ 100 keV into the BOX layer 106.In this embodiment, the final n-type diffusion source 116 and p-typediffusion source 120 dopant concentration in the BOX 106 is preferablyat least 10²⁰ cm⁻³. As will be described in greater detail below, thediffusion sources 116, 120 provide a source of dopant atoms that candiffuse into the wells 112, 114 respectively to create a retrogradedopant profile.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisesthreshold voltage (vt) adjust implants 122, 124 as shown in FIG. 5. Thethreshold voltage adjust implants 122, 124 adjust the threshold voltageof the PMOS 132 and NMOS 134 devices either upwards or downwards in amanner known in the art. The threshold voltage adjust implants 122, 124comprise, in this embodiment, a PMOS gate adjust 122 implant of BF₂ at adose of approximately 5e12 to 1e13 @ 25-35 keV and an NMOS gate adjust124 implant of Arsenic at a dose of approximately 5e12 to 1e13 @ 35-50keV. The PMOS gate adjust 122 and the NMOS gate adjust 124 modify thedopant concentration in the gate region of the PMOS 132 and NMOS 134devices so as to adjust the resultant threshold voltage of the PMOS 132and NMOS 134 devices to a desirable level.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisesformation of a gate stack 136 as shown in FIG. 6. The gate stack 136comprises a gate oxide 126, sidewalls 140, a nitride layer 142, anddoped polysilicon 144. The gate oxide 126 in this embodiment comprises alayer of silicon dioxide approximately 50 Å thick. The gate oxide 126electrically isolates the n-well 112 and p-well 114 regions of theepitaxial silicon 110 from overlying conductive layers that will bedescribed in greater detail below. The sidewalls 140 comprise silicondioxide that is grown and subsequently anisotropically etched in a knownmanner to create the structures illustrated in FIG. 6. The sidewalls 140electrically isolate the gate stack 136 from source/drain conductivelayers and facilitates formation of source/drain extensions in a mannerthat will be described in greater detail below. The nitride layer 142comprises a layer that is substantially silicon nitride approximately450 Å thick emplaced in a known manner. The nitride layer 142 inhibitssubsequent passage of Boron from the p+ polysilicon layer 144. The dopedpolysilicon 144 comprises heavily p-type doped polysilicon for the PMOS132 device and heavily n-type doped polysilicon for the NMOS 134. Thedoped polysilicon 144 provides a reduced work function for the gates ofthe PMOS 132 and NMOS 134 (FIG. 7) and thus a lower contact resistanceand corresponding faster device response.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisesformation of the source 146 and drain 150 as shown in FIG. 6. The source146 and drain 150 are formed by implanting BF₂ with a dose ofapproximately 2e15/cm² @ 15 keV for the PMOS 132 and As with a dose ofapproximately 2e15/cm² @ 10 keV for the NMOS 134. As can be seen fromFIG. 6 the implantation of the source 146 and drain 150 is partiallymasked by the gate stack 136 and results in source/drain extensions 152.The source/drain extensions 152 are lower concentration regions of thesource 146 and drain 150 that partially extend under the sidewalls 140.The source/drain extensions 152 reduce the peak electric field under thegate and thus reduce hot carrier effects in a known manner.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisesformation of a conductive layer 154 (FIG. 7). In this embodiment, theconductive layer 154 comprises a layer of metallic silicide (titaniumsilicide or cobalt silicide) emplaced in a well known manner. Theconductive layer 154 is placed so as to be in physical and electricalcontact with the source 146, the drain 150, and the doped polysilicon144 of the gate stack 136. The conductive layer 154 interconnects theCMOS 130 with other circuit devices on the SIMOX wafer 102 in a knownmanner.

The method of forming the SOI CMOS with reduced DIBL 100 then comprisesformation of a passivation layer 156 (FIG. 7) overlying the structurespreviously described. In this embodiment, the passivation layer 156comprises a layer of oxide, BPSG, or polysilicon approximately 3000 Åthick formed in a known manner. The formation of the passivation layer156 involves a high temperature process.

The n-type diffusion source 116 and the p-type diffusion source 120previously implanted into the BOX layer 106 in the manner previouslydescribed serve as solid-sources for dopant diffusion. When thepassivation layer 156 is formed on the SIMOX wafer 102 with attendantheat steps, dopants contained in the n-type 116 and the p-type 120diffusion sources will outdiffuse into the epitaxial silicon 110,creating a thin, highly doped retrograde profile region 160 as shown inFIG. 7. In the case of the p-well 114, the retrograde profile region 160will comprise boron and, in the n-well 112, the retrograde profileregion 160 will comprise phosphorus. The retrograde profile region 160layer will act as a punchthrough prevention layer to control DIBL.

FIG. 8 shows the net dopant profile in a vertical outline in the middleof the channel region. The boron concentration increases from 9.0e17/cm³to 2.0e18/cm³, which is nearly a 120% increase, at the BOX 106/siliconsubstrate 104 interface. FIG. 9 shows the dopant profile in the source146 and drain 150 regions. The source 146 and drain 150 implants in thisembodiment of the SOI CMOS with reduced DIBL 100 reach close to the BOXlayer 106 as can be seen from FIG. 9. As such the source 146 and drain150 implants will compensate the outdiffused dopants from the n-type 116and p-type 120 diffusion sources in the retrograde profile region 160close to the interface of the BOX 106 and the silicon substrate 104.This will reduce the junction capacitance of the SOI CMOS with reducedDIBL 100 even further as compared to a process with halo implants.

The dopants contained within the retrograde profile region 160 will alsocreate recombination centers near the BOX 106/silicon substrate 104interface. These recombination centers are an added benefit in the SOICMOS with reduced DIBL 100 since the recombination centers tend toreduce the floating body effects in the SOI CMOS with reduced DIBL 100.

Hence, the process of the illustrated embodiment provides a method inwhich a retrograde doping profile can be created in thin semiconductoractive areas such as the active areas used in silicon-on-insulator (SOI)applications. The process of the illustrated embodiment does notsignificantly add to the processing of the device as only discreteimplantation steps are required and the diffusion is obtained throughthe additional thermal processing of the device. Thus, retrogradeprofiles can be created in a manner that does not significantly increasethe processing costs of the device.

Although the preferred embodiments of the present invention have shown,described and pointed out the fundamental novel features of theinvention as applied to those embodiments, it will be understood thatvarious omissions, substitutions and changes in the form of the detailof the device illustrated may be made by those skilled in the artwithout departing from the spirit of the present invention.Consequently, the scope of the invention should not be limited to theforegoing description but is to be defined by the appended claims.

What is claimed is:
 1. A method for creating complementary semiconductortransistor devices on a silicon wafer comprising: forming a buriedsilicon dioxide (BOX) insulation layer in the silicon wafer; growingepitaxial silicon atop the silicon wafer so as to form an active layerabove the buried insulation layer; implanting n-type or p-type dopantsinto the active layer to create n-wells and p-wells respectively so asto form complementary device regions in the active layer; implantingn-type and p-type dopant atoms through the complementary device regionssuch that the dopant atoms come to reside within the BOX insulationlayer underlying the device regions so as to create aborophosphosilicate glass (BPSG) within the BOX layer; implanting dopantatoms into gate regions of the device regions so as to adjust thethreshold voltage of the transistor devices; forming a gate stack on theactive layer adjacent the gate regions; and implanting dopant atoms intothe device regions such that the dopant atoms come to reside within thedevice regions adjacent the gate regions so as to form source and drainregions and wherein the gate stack substantially inhibits penetration ofthe dopant atoms into the gate regions of the device regions.
 2. Themethod of claim 1, further comprising: forming conductive paths thatelectrically connect to the source, drain, and gate regions; and forminga passivating layer overlying the conductive paths wherein forming thepassivating layer overlying the conductive paths induces the dopantatoms contained within the BPSG within the BOX layer to outdiffuse intothe device regions to thereby create a retrograde dopant profile withinthe device regions.
 3. The method of claim 2, wherein the retrogradedopant profile within the device regions has a peak concentrationsubstantially adjacent the interface of the BOX and the siliconsubstrate.
 4. The method of claim 3, wherein the retrograde dopantprofile within the device regions provides the transistor devices withimproved resistance to drain-induced barrier lowering (DIBL).
 5. Themethod of claim 3, wherein the retrograde dopant profile within thedevice regions provides recombination centers to reduce floating bodyeffects in the transistor device.